1. Field of the Invention
The present invention relates to a color image forming apparatus of electrophotographic type such as a color copying machine, a color printer and the like.
2. Related Background Art
In some color image forming apparatuses, latent images corresponding to various color components are formed on an image bearing member and such latent images are developed by developer having colors corresponding to the latent images to obtain color toner images. Whenever each toner image is obtained, the toner image is transferred onto a transfer material held by a rotating transfer drum. By repeating such transferring operations by several times corresponding to the number of color toner images, a multi-color image is obtained. In this case, as shown in FIG. 5, by firmly assembling an electrophotographic photosensitive drum (image bearing member) 4a to a body 9 of the image forming apparatus with high accuracy, disarrangement or vibration of rotation of the photosensitive drum 4a is prevented, thereby obtaining the color image with higher quality.
Further, since the photosensitive drum 4a must have a relatively large diameter (at least a half of a diameter of a transfer drum 3) in order to improve the durability of the photosensitive drum 4a, the diameter of the photosensitive drum 4a was 50 mm or more.
However, in the conventional combination of the photosensitive drum 4a and the transfer drum 3, regarding a plurality of developing devices 5 (i.e., magenta developing device 5M, cyan developing device 5C, yellow developing device 5Y and black developing device 5K), after development for a certain color was finished, a mechanical switching operation for bringing the next developing device to the photosensitive drum 4a to perform development for a next color was effected before the previous transferring operation is completely finished. That is to say, it was designed so that a distance (X.sub.1) between a latent image forming position (where image information light .lambda. is incident to a surface of the photosensitive drum 4a ) and a transfer position (where the photosensitive drum 4a is opposed to the transfer drum 3) becomes greater than a value obtained by subtracting a length (X.sub.3) of the transfer material 2 wound around the transfer drum 3 from a peripheral length (X.sub.2) of the transfer drum 3, that is, (X.sub.2 -X.sub.3)&lt;X.sub.1. With this arrangement, unless the previous developing device is changed or switched to the next developing device during the transferring operation, the next latent image formation cannot be effected in time. Thus, since the developing devices were rotated during the transferring operation, vibration and/or shock were generated, thereby affecting a bad influence upon the image. Incidentally, in order to prevent such vibration, it is desirable to rotate the developing devices while the transfer drum is being rotated idly without the transferring of the image. In this case, however, the number of revolutions of the transfer drum was increased in accordance with the number of the color toner images, thereby increasing the image forming time.